This invention relates to digital data transmission systems for transmitting Baudot code characters each having start and stop bits in the form of a MSK digital data signal having a stop-start triangle corresponding to adjacent stop and start bits of two adjacent Baudot characters and more particularly to a MSK digital data synchronization detector for such systems.
MSK is an abbreviation for "minimum shift keying" and may be defined as a signal where the transmitted wave is a phase continuous frequency shift waveform. For each unit time interval the instantaneous frequency is a constant being either a high frequency or a low frequency relative to a center frequency. The high frequency is such as to advance by one phase revolution relative to the carrier frequency in four unit time intervals. The low frequency is such as to fall behind by one phase revolution relative to the carrier frequency in four unit time intervals.
In the past, Baudot code characters each having start and stop bits have been transmitted in the form of a CSK modulation. CSK is an abbreviation for "compatible shift keying". A CSK data stream is a continuous frequency shift modulation waveform with the same upper and lower frequencies as MSK modulation. The only difference between MSK and CSK modulation is in the method in which the data in modulated on to the carrier. In MSK modulation each data bit selects the sign of an amplitude modulated carrier phase in the form of EQU cos w.sub.o t cos w.sub.m t (1)
where -T-- t&lt; T, 4 w.sub.m T= 2.pi. , w.sub.o is equal to the carrier frequency in radians, w.sub.m is equal to the modulating frequency in radians, T is equal to a unit time interval and t is equal to time. In CSK modulation each data bit selects the instantaneous frequency of the transmitted signal of one bit time.
One prior art procedure for CSK modulation signal detection and, hence, synchronization detection, is to search in various carrier phases in bit times looking for stable tracking behavior and low orthogonal signals. Stable tracking behavior and low orthogonal signals indicate signal presence and also synchronization. The direct data decision procedure is to decide each CSK bit separately.
An alternate procedure employed in the prior art is to use MSK decisions and to convert 6 MSK decisions into 5 CSK decisions.
One of the disadvantages of the above-mentioned prior art detectors is that they require a high signal-to-noise ratio, or multiple searches to acquire synchronization at the same signal-to-noise ratio. Also, the error rate for the above prior art procedures require several db (decibel) more signal-to-noise ratio for the same probability of error at Baudot character error rates of 10.sup.-.sup.3 or lower.